The present invention is directed to hydraulic control systems, and pertains more particularly to an improvement in control systems such as in application Ser. No. 583,366 entitled Part Throttle Control - Pump Override, now U.S. Pat. No. 3,963,378, assigned to the assignee hereof and having a plurality of variable displacement pumps and means for automtically adjusting the displacement of the pumps to engine output.
Many hydraulically actuated implements, such as a hydraulic excavator, normally employ a plurality of variable displacement pumps for supplying the high-pressure hydraulic fluid necessary to power the many motors employed in manipulating the excavator bucket and linkage. Reference is made to U.S. Pat. No. 3,841,795 issued Oct. 15, 1974 to Ferre et al., assigned to the assignee hereof, for background material pertinent to the subject invention. The above patent was developed to overcome certain problems of the prior art and functions very satisfactorily so long as a single predetermined engine speed or engine normal operating speed is selected and maintained.
Earth-moving machines, such as the hydraulic excavator, are normally designed to have a predetermined normal operating speed of the engine during normal operation of the machine. With such machines the engine throttle is normally set at a predetermined position when the machine is in normal operation.
Frequently, it is desirable to operate the vehicle at other than normal engine speed. This becomes a problem when the hydraulic system is equipped with a control system such as that of U.S. Pat. No. 3,841,795. With such a system, as soon as the engine speed is reduced, the differential pressure across the underspeed valve orifices would respond as though the engine was running slower due to lugging, and signal the pumps of the system to stroke to a lower output. Thus, one would be unable with that system to operate the vehicle with the engine at a lower speed and at the same time obtain full pump flow from the pumps.
Copending application Ser. No. 583,366 entitled Infinitely Variable Orifice for Part Throttle Control by R. G. Butler, and of common assignment herewith discloses a system wherein the response to the underspeed is made infinitely variable with engine throttle control. However, it has been learned that it is desirable to deter response of the underspeed valve until the engine has attained a certain minimum speed. Thus, the present invention has been devised to accomplish this end.